My son started this this year. He's been going up a grade for math since 1st grade. This year he is in 5th grade, so he now goes to Middle School for 6th grade math. It's worked out great so far (they've been back 3 weeks now) - he gets transported to and from MS for 2nd period math (the school district provide a cab). Apparently, when the school district normally transport (usually MS kids to HS), they only provide transportation one way, but thankfully our principal was able to get 2 way transport. I think it also helps that the MS is only about a 5 minute drive from the Elementary school
My son is a young 5th grader (turned 10 in June) - and also short for his age - but has been doing great, no problems with fitting in. I think that an accelerated 6th grade class would have been a better fit, but for now, I'm just happy that the district are letting him do this. Next year, the MS will be implementing a full time gifted program, so he should get plenty of challenge then
